In the motor vehicle, claw pole generators are predominantly employed at present to generate energy. Claw pole generators are rotary current machines, which can be described approximately by the principle of the synchronous salient pole machine. Typically, the three-phase output current of the generator is rectified by a passive B6 diode bridge; FIG. 1 shows such an arrangement. The generator is regulated by the exciter current such that the output voltage UB of the generator is set to the desired value independently of the rpm and, within the context of the power handling capacity of the generator, also independently of the load. With a generator and the associated rectifier circuit as shown in FIG. 1, the power does not begin to be produced until after a certain rpm, the so-called startup rpm, is reached. At rpm levels less than the startup rpm, the generator voltage is lower than the battery voltage. In that case, no power can be fed into the on-board electrical system via the B6 rectifier bridge.
One possibility of increasing the power of a claw pole generator is to increase the exciter current. Given the usual machine designs for the claw pole generators employed in the motor vehicle, however, this leads to pronounced magnetic saturation phenomena. These saturation phenomena can reduce the power increase substantially. Furthermore, a marked increase in the exciter current during continuous operation leads to a thermal overload on the generator, so that this method can be employed for only a limited time.
Another point of departure for increase in power is to vary the number of stator windings. Increasing the number of stator windings leads to a reduction in the startup rpm and to an increase in the outset power in the low rpm range. However, in that case, the outset power in the middle and high rpm range is reduced markedly. Reducing the number of stator windings does lead to a power increase in the high rpm range, but at the same time the power in the lower rpm range is reduced, and the startup rpm is increased. For these reasons, varying the number of stator windings does not lead to the goal of increasing power over a wide rpm range.
In German Patent Application DE-P 197 33 221.8, it is shown how the power characteristic of the generator can be varied in an advantageous way, with the aid of a high-level setting controller or low-level setting controller connected downstream of the generator and by varying the numbers of stator windings. If, as described in German Patent Application DE-P 198 45 569.0, then number of stator windings is reduced and a high-level setting controller is connected downstream of the generator with a diode rectifier, the result is an increase in power in the middle and high rpm range. In the low rpm range, the voltage of the generator is raised to the voltage level of the on-board electrical system, with the aid of the high-level setting controller.
A substantial disadvantage of rectification of the three-phase generator currents with the aid of a passive B6 bridge is that the amount and phase of the phase current or the pole wheel angle cannot be used as a controlling variable for regulating the generator. In that case, the star voltage is fixedly predetermined by the on-board electrical system voltage, and the phase angle between the phase current and the star voltage is, with good approximation, xcfx86=0 at all operating points. The machine can therefore not be guided optimally in all operating points with regard to the outset power, efficiency and regulation dynamics.
If the generator is operated together with a pulse inverter, this limitation no longer exists. References are also known that address the use of a claw pole generator in conjunction with a pulse inverter.
In German Patent Application DE-P 197 33 212.9, a method for regulating a generator that can be driven by an internal combustion engine is described. This reference indicates the possibility of using two regulation ranges, that is, the basic rpm range at low rpm and the field attenuation range at higher rpm. No regulating method is described. The regulation of the exciter current is not mentioned. The lack of inclusion of the exciter current, however, leads to machine or generator guidance that is not optimized in terms of efficiency and power.
U.S. Pat. No. 5,648,705 or European Patent Disclosure EP 0 762 596 A1 describes a method in which the power enhancement in the lower rpm range by varying the angle between the stator current and the pole wheel voltage is addressed. A well-developed regulation concept is not described. If maximum outset power is not needed in the low rpm range, then the three-phase current of the generator is set to the desired output voltage, with the aid of a passive diode bridge and with regulation solely of the exciter current. For the passive diode bridge, the parasitic diodes of the MOSFET switches of the current inverter circuit are used. However, regulation in the middle and higher rpm ranges is not discussed. Optimal machine guidance in terms of power and efficiency in the middle and higher rpm range at full and partial load is therefore not achieved.
In U.S. Pat. No. 5,663,631, a generator regulation is described in which the exciter current IE of the machine is ascertained by a regulator, solely with the aid of the difference xcex94UB=UBsollxe2x88x92UBist between the set-point value UBsoll and the actual value UBist of the outset direct voltage. The pole wheel angle xcex4 of the machine is set for maximum outset power to xcex4=90xc2x0. At the same time, the possibility is mentioned of operating the machine at an operating point that is optimized for the sake of efficiency by means of a suitable choice of the pole wheel angle. Mention is made of the fact that the precise setting of the pole wheel angle can be done as a function of the exciter current. There is no description of the way in which the pole wheel angle has to be varied precisely as a function of the exciter current.
An essential definitive characteristic of this method is that determining the exciter current is dependent only on the regulation difference xcex94UB. A correction of the pole wheel angle, which is mentioned in the patent, is done only as a function of the exciter current.
In contrast to the known method, in the claims methods are defined that for each rpm point and each desired generator power can select the exciter current and the phase current freely in terms of amount and phase. This opens up the possibility of selecting the most suitable operating state for current requirements, from among all the possible operating states of the machine or generator, and accordingly setting the operating point of whatever is most favorable for the demands made. In the present patent application, the desired outset power influences not only the set-point value of the exciter current but also directly influences all the control parameters of the machine that are relevant to guiding the machine.
The method known from U.S. Pat. No. 5,663,631 has the additional disadvantage, compared with the present patent application, that the regulation dynamics are determined by the exciter circuit. With respect to its behavior in the event of major load changes, it therefore has the same disadvantage as rectification with a passive B6 diode bridge. Thus load changes are regulated dynamically only in accordance with the high exciter time constants. In the present patent application, however, the amount and phase relationship of the phase currents are additionally utilized for the sake of dynamic high-quality regulation of major load changes.
The method and apparatuses of the invention for regulating a generator have the advantage that both the available outset power and the efficiency of the generator can be increased substantially over a wide rpm range, compared to a generator with passive diode rectification.
In addition, by the method described, a marked improvement in regulation dynamics is attained, for instance in the event of a load dump. In modern generators, the protection against overvoltages upon a load dump is done with the aid of the rectifier diodes, embodied as Zener diodes. If the generator is intended to generate a relatively high on-board electrical system voltage, such as UB=42 V, then considerable problems arise in manufacturing Zener diodes with relatively high voltage. To achieve protection against overvoltages upon a load dump even at higher generator voltages, such as UB=42 V, other methods for voltage limitation must therefore be chosen. The method presented here advantageously combines the increase in power and efficiency of the generator with an effective protection against overloads upon a load dump.
These advantages are attained by a method which comprises a total of four different regulation ranges for regulating the generator. One of the four regulation ranges is selected, depending on the given demands and peripheral conditions, such as the desired outset power and the given generator rpm. In this way, compared to operation with passive rectification, a substantially more favorable machine guidance is possible, which leads to an increase in the available outset power, in the efficiency, and in the regulation dynamics.
The methods claims are advantageously usable for both synchronous full-pole and synchronous salient pole machines and in generators that are based on similar functional principles, such as claw pole generators. The phase windings of the machine can be wired both in a Y-type circuit and in a delta-type circuit. A method is described, taking an idealized synchronous salient pole machine as an example. In machines that have similar functional principles or that have non-negligible additional effects, such as a pronounced non-linear magnetic behavior, the described method can be employed with the aid of similar considerations.
Further advantages of the invention are attained by the provisions recited in the dependent claims.